Wavelength division multiplexing (WDM) devices are becoming increasingly important in fiber-optics sensing systems and optical communication systems to enhance transmission capacity and application flexibility. Several WDM devices are known including Mach-Zehnder interferometers (B. H. Berbeck, C. H. Henry, N. A. Olsson, K. J. Orlowsky, R. F. Kazarinow, and B. H. Johnson, "Integrated Four-Channel Mach-Zehnder Multi/demuliplexer Fabricated With Phosphorous Doped SiO.sub.2 Waveguide On Si," J. Lightwave Technol., Vol. 6, pp.1011, 1988), arrayed waveguide gratings (H. Takahashi, S. Suzuki, K. Kato, and I. Nishi, "Arrayed-Waveguide Grating For Wavelength Division Multi/demuliplexer With Nanometer Resolution," Electron. Lett., Vol. 26, pp. 87, 1990), planar spectrometers (J. B. D. Soole et al, "Monolithic InP-based Grating Spectrometer For Wavelength Division Multiplexed Systems At 1.5 .mu.m," Electron. Lett., Vol. 27, pp. 132, 1990), and directional couplers (H. Sakata, S. Takeuchi, "Grating-Assisted Directional Coupler Filters Using AlGaAs/GaAs MQW Waveguides," IEEE Photonics Technol. Lett., Vol. 3, pp.899, 1991). For anticipated devices to be used in for example fiber-to-home applications, large-channel fanout, low loss and compatibility with fiber connections, laser sources and detectors will be advantageous features of WDM devices. Dual-functionality of wavelength selection and beam splitting are also desirable properties. Bragg grating devices which posses the advantage of high diffraction efficiency, high wavelength selectivity and high angular selectivity have received considerable attention in recent years, see for example R. T. Chen, H. Lu, D. Robinson and T. Jannson, "Highly Multiplexed Graded-Index Polymer Waveguide Hologram For Near-infrared Eight-Channel Waveguide Division Multiplexing," Appl. Phys. Lett., Vol. 59, pp. 1145, 1991, and C. H. Henry, R. F. Kazarinov, Y. Shani, R. C. Kistler, C. Plo, K. J. Orlowsky, "Four Channel Wavelength Division Multiplexers And Bandpass Filter Based On Elliptical Bragg Reflector", J. of Lightwave Technol., Vol.8, pp. 748,1990.
U.S. Pat. No. 3,498,693 issued to Fein et al. is directed to an optical filtering device using an etalon having two spaced mirrors that are partially transmitting. By varying the distance or tilt between the mirrors the wavelength selection is achieved for wavelengths satisfying the resonance condition.
U.S. Pat. No. 5,119,454 issued to McMahon teaches an optical wavelength division multiplexer having an etalon-like structure defined by a pair of opposed mirror surfaces with a planar diffraction grating formed on one of the reflective mirror surfaces. Wavelength selection in this device is achieved by tuning of the resonant condition along the length of the etalon-like structure and the diffraction grating is used to change the direction of propagation of the light beam.
U.S. Pat. No. 5,144,498 issued to Vincent is directed to a variable wavelength light filter and sensor. The device uses an etalon structure in which the wavelength filtering function is achieved by utilizing the dependence of the resonance condition along the length of the etalon.
For high density and large fan-out WDM applications, the number of gratings in a superimposed grating structure is constrained by signal-to-noise ratio, see (V. Minier, A. Kevorkian, J. M. Xu, "Diffraction Characteristics Of Superimposed Holographic Gratings In Planar Optical Waveguide", IEEE Photonics Tech. Lett. Vol. 4(10), pp. 1115, 1992), and by the limited refractive index modulation depth of the material; on the other hand, cascaded gratings span a large physical dimension. Polarization sensitivity is also a drawback of known grating based WDM devices.
Therefore, it would be very advantageous to provide a WDM device exhibiting wavelength selectivity, high diffraction sensitivity, high angular selectivity and beam splitting that is polarization insensitive.